1. Technical Field
Various embodiments of the present disclosure generally relate to a nonvolatile memory device, and, more particularly, to an EPROM device for storing multi-bit data and a read circuit of the EPROM device.
2. Related Art
Semiconductor memory devices are typically classified into random access memory (RAM) devices and read only memory (ROM) devices according to data volatility thereof. The RAM devices are volatile memory devices that require power to retain their data (e.g., binary information) and lose stored data when the power supply is interrupted. In contrast, the ROM devices can retain stored data even when the power supply is interrupted. ROM devices may also be classified into programmable ROM (PROM) devices and mask ROM devices according to its data input method (e.g., data program method). The PROM devices may be a form of ROM that can be programmed by customers (i.e., users) even after fabrications of the PROM devices have been completed. The Mask ROM devices may be programmed during the fabrications thereof using implantation masks manufactured on the basis of data requested by users. Examples of the PROM devices may include one-time PROM (OTPROM) devices, erasable PROM (EPROM) devices and electrically erasable PROM (EEPROM) devices. Once the EPROM devices are programmed, the programmed data of the EPROM devices cannot be electrically changed. The programmed data of the EPROM devices can be erased by a physical means such as UV irradiation.
Nonvolatile memory device such as the EPROM device may employ an NMOS transistor or a PMOS transistor as a cell transistor. If a PMOS transistor is used as the cell transistor of the nonvolatile memory device, the PMOS transistor may have a turn-off state as an initial state, and may have a turn-on state as a programmed state. A read operation of the PMOS transistor may be executed by sensing whether a program current flows through the PMOS transistor.